It's like the current world financial system. You have the Euro, Yuan, Yen, Ruble, Pound, (alts made by other entities) etc, and they are all primarily supported by and traded with the USD (Bitcoin). It's not like having these currencies makes more USD avaliable (while more USD is made avaliable through other means). It just offers more mediums.
The altcoins increase the likelihood of the Bitcoin ecosystem reaching individuals. If Bitcoin was deflationary then its reach would be limited to how those coins distributed in the ether. Speaking of ether Just look at Ethereum. I am hearing that coin being mentioned inside communications within a company I work for that has nothing to do with BItcoin. That altcoin just exposed the blockchain by being a derivative of Bitcoin.
To your second point I somewhat agree accept for you saying that the other currencies are just mediums. The other currencies allow populations of people to expand or contract based on their value. That is a very powerful thing.
All in all there is a reason why the current fiat scheme is at odds right now. My argument is that we should not be copying it if Bitcoin wants to survive. People may not agree with the parallels that I present but that is ok.